Want to add subtitles or captions to your YouTube videos? Webapp CaptionTube can walk you through the process, helping create captions which you can upload in XML format straight onto YouTube.
The webapp is straightforward, though showing its beta status; signing in and granting permissions is a tad fiddly, and video buffering before adding captions took a fair while.. If you want to annotate a video you didn't upload, you can email the caption file to the video owner, though of course they might choose to ignore your hard work. CaptionTube is free to use, requires a Google account.